Multi-axis light beam-type sensors such as disclosed in U.S. Pat. No. 4,309,696, have been adopted as a method of ensuring the safety of an operator in situations such as where the operation of a press machine is carried out manually by the operator.
With such a multi-axis light beam-type sensor, there is a region where it is dangerous for an operator to be present when the press machine is operating, and a security region where it is safe to be present. At the boundary to such regions, a plurality of light emitting elements and light receiving elements are arranged facing each other so that light beams from the light emitting elements are emitted towards the light receiving elements. When a portion of an operator's body blocks at least one of the light beams so that an output from the light receiving element is not generated, it is judged that an operator has entered the danger region, and the press machine is stopped.
In this case, because the spacing of the light emitting elements and the light receiving elements aligned vertically is set narrow (of the order of 2 cm), and the light beams from the light emitting elements are spread to a certain degree, they are incident on not only the corresponding light receiving elements but also on other neighboring light receiving elements. In order to prevent generation of a light receiving element output due to the incidence of a light beam from a non corresponding light emitting element, the frequency of the light beams for each light emitting element is made different, and a filter which extracts only the light beam frequency of the corresponding light emitting elements is provided on the respective light receiving element side.
With the above-mentioned circuit construction however, an increase in the number of light emitting elements and light receiving elements results in an increase in the number of frequencies used, thus requiring an increase in the corresponding number of filters.
In order to solve this problem, a scanning drive system has been considered which scan drives the corresponding light emitting elements and light receiving elements with successive switching, so that the respective light emitting elements emit light in continuous succession at different times, with the process being repeated in cycles.
With the conventional scanning circuit however, which generates a scanning output for a driven object such as a light emitting element or a light receiving element, an abnormality such as the simultaneous generation of a plurality of scanning outputs causing the simultaneous generation of a plurality of light beams, cannot be detected.
It is an object of the present invention to address the above-mentioned problems, and provide a fail-safe scanning circuit which can detect an abnormal condition such as the simultaneous generation of a plurality of scanning outputs for driving driven objects, as well as providing a fail-safe multi-axis light beam-type sensor using such a fail-safe scanning circuit.